Best way to insulate a vaulted gambrel roof?
We are in the planning phase of a project in Conifer, CO (8300’). Building a shop with ICF walls/floor and open span Glue-lam rafters from Starwood rafters. Living space will be on the second level in the vaulted ceiling. Our plan is to live in the shop while we build the house, and eventually use it as a space for guests.
I’m struggling to find the best, most cost effective way to frame the roof over top of the rafters. We are considering SIPs more than anything but cost is pretty crazy. We have looked at OSB sandwich panels, and the Ray-Core foam panels with integrated 2×8 studs.
Would we be better off framing with purlins, sheathing, and exterior insulation board with offset seams? I’ve considered spray foam but would like to avoid it if possible.
What other insulation methods should we be looking into? The property will be off grid so R value and airtightness are important to maximize efficiency. We want to build it right, but spending too much on the barn will take away from our home budget.
Also, what other issues/mistakes do you see in our early plans?? I’m sure there are plenty.
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Deleted
Jackson,
A couple of thoughts.
- The open stair inside seems like a bit of a problem. It interrupts the shop space below. It also means you have to traverse the shop to get to the house above, which practically isn't great, but depending on what the shop is used for may be a code violation. I would consider an exterior enclosed stair. This project deals with the situation well:
https://www.kaplanthompson.com/project/white-mountain-view/
The glu-lam rafters are cool, but much easier from both structure and insulation perspective (and less expensive) would be to use gambrel trusses.
Thanks for that link, that project turned out awesome. I wish there were more photos of how the stairs are set up.
We are still very up in the air on the stairs. At first our goal was to keep them outside of the conditioned space to simplify things but we are still trying to figure out what the planning and zoning people will allow.
We have looked at some gambrel web trusses but they take up far more interior space than the glu-lams. The living space has a sleeping loft as well for the kiddos and I don’t think that would be an option with trusses. Also, depending on the roof structure, we are able to space the rafters 8-12’ on center. So yes they are more expensive per unit, but we wouldn’t have to buy very many. Also, we just like the look of them compared to a web of 2x4s and metal plates!
Jackson,
You may well be right that the glu-lams are worth it for purely architectural reasons.
I agree with Akos that a vented roof with batt insulation would be my first choice. I'm not sure how to secure the horizontal purlins on the steep part of the roof, but he probably does and will weigh-in
Why not run purlins between the gluelams (say 2x12 or 14"/16" I-joist), fill these with fluffy to get the R value you want. Over these run vertical 2x4s on flat to form the vent space. Vent intake at the bottom of the gambrel roof and exhaust at the ridge. Simple vented roof that works in any climate built with cheap materials without any foam. Pretty close to standard post barn build.
You do need a solid warm side air barrier which can be one of the fancier membranes or 6mil poly. In either case, it needs to be well though out where it intersects any structural bits. If you want, you can also run housewrap with taped seams over the purlins on the outside as you secondary air barrier. The nice part about the exterior housewrap is that is much easier to get air barrier continuity but a bit of a pain to install (bit easier with purlins as you can walk on the framing).
I have been thinking about the purlins with insulation in the cavities but I can’t wrap my head around what the framing would look like. Would the purlins sit on top of the glue lams or run between them? How would they attach? Would it be worthwhile to do 2x10s or 2x12s with an exterior layer of rigid foam (with taped seams as a secondary air barrier) to reduce bridging? Then with the 2x4s over top of the foam, will that need some sort of underlayment between the steel roof and the foam?
Also, how would the insulation be held in place? I assume you mean bat insulation rather than blown in cellulose…? Do I need to do my warm side air barrier and interior sheathing/drywall in place before I insulate?
Sorry for my severe ignorance here. This is all new and a lot to take in. I’m sure these are dumb questions with very simple answers to the people on this forum but it’s all Greek to me!
I think it would make sense to leave those gluelams exposed. Would definitely look much nicer.
In that case, the purlins would over the gluelams. This makes interior air barrier continuity easier, you can put a strip of membrane (house wrap or vapor retarder) over the gluelams before setting the purlins. You can then tape the warm side vapor retarder to this strip for air barrier continuity.
On the low slope side it is pretty straight forward. The purlins get toe nailed same as a rafter with blocking to prevent racking. Depending on your local wind/seismic conditions you might need some hardware to tie them to the gluelams.
For the steep section, depending on the gluelam spacing you will need some additional blocking along the purlins for vertical support.
You can do it with batts or dense pack with fluffy. Either works as well. Dense packing is normally done through something like insulweb or intello before the drywall is installed.